directions.hs

type Coord = (Int, Int)
type Direction = Coord -> Coord
type Boundary = Coord -> Bool

north :: Direction
north (row, col) = (row - 1, col)

south :: Direction
south (row, col) = (row + 1, col)

east :: Direction
east (row, col) = (row, col + 1)

west :: Direction
west (row, col) = (row, col - 1)

isInBounds :: Boundary
isInBounds (row, col) = and [row >= 0, row < 8, col >= 0, col < 8]

-- Starting Coord is not included in the walk
walkDirection :: Boundary -> Direction -> Coord -> [Coord]
walkDirection bound dir crd = takeWhile bound $ tail $ iterate dir crd

-- Some sample REPL exchanges:
-- walkDirection isInBounds north (3,3)
-- => [(2,3), (1,3), (0,3)]
-- Directions compose nicely:
-- walkDirection isInBounds (north . west) (3,3)
-- => [(2,2), (1,1), (0,0)]